The present invention relates to gas discharge tubing power supplies and the like.
Power supplies are typically used in powering gas discharge lighting, to convert a low impedance, low voltage power source, such as a 120 Volt 60 Hz AC wall outlet, into a higher voltage source suitable for connection transformer is used to step up the voltage of the 120 Volt AC source. The 120 Volt AC source is connected to the primary winding of the transformer and the secondary winding of the transformer is connected to the gas discharge lamp.
A neon sign (hereinafter also called xe2x80x9cneon tubingxe2x80x9d), is one example of a gas discharge lamp. Neon signs typically use a transformer (hereinafter also called a xe2x80x9cneon transformerxe2x80x9d) to illuminate the sign. The following discussion of the background and the invention will refer to power supply circuits used for neon signs, however, it will be understood that principles of the present invention have application to power supply circuits for other gas discharge tube lamps as well.
A primary concern with known neon power supplies, is the potential that a ground fault from the high voltage outputs of the power supply can create substantial current flows, potentially causing fires if the ground fault creates an arc involving flammable materials. A potentially dangerous ground fault current may occur anytime there is a relatively low impedance path from one of the high voltage output leads of the neon power supply to ground. Such a path may be formed if a neon sign is carelessly installed so that one of the output leads connected to the sign is in contact with a low impedance in a window frame, doorway, or other ground-connected relatively low impedance.
To detect ground fault current, it is typically necessary to couple a ground fault detection circuit to the secondary winding of the power supply transformer, and/or to the neon sign itself. Specifically, the ground fault detection circuit may be coupled between a path to ground, and either a center tap of the secondary winding of the transformer, and/or a return point located near the electrical mid-point of the neon tubing. If there is a secondary ground fault, the transformer circuit automatically interrupts power.
Troubleshooting a neon sign for ground faults is difficult, because ground faults are not always visibly detectable. Often, it is necessary to carefully measure currents flowing through various connections to determine the location of the fault, which can be an arduous process. If difficulty in locating a fault may tempt the installer to conclude that the secondary ground fault interruption circuitry is malfunctioning, and try to defeat the ground fault circuitry.
Thus, there is a need for circuitry which enables a gas discharge tubing installer to identify and pinpoint the location of a ground fault quickly and accurately, to speed installation and minimize the temptation for tampering with the ground fault detection circuitry.
The circuitry described in the above referenced U.S. patent application Ser. No. 08/838,060 includes several features for preventing the ground fault detection circuitry from being inadvertently or deliberately defeated. A difficulty with this and other known secondary ground fault interrupting circuits, is that none can reliably detect a ground fault reliably when the center of the gas tube load is connected to earth ground. This is due to cancellation of the opposite and nearly equal currents entering the ground node. The net resulting current into/out of ground may not be sufficient to activate a secondary ground fault detection circuit connected between a midpoint of the transformer secondary and ground. Thus, the ground fault detection features of these circuits can often be defeated by grounding the mid-point of the load.
For example, FIG. 1 illustrates a neon transformer circuit 10 with a secondary ground fault interrupter, such as that described in the above-referenced U.S. patent application Ser. No. 08/838,060, connected to a load which is grounded at its mid-point. The neon transformer includes a primary winding 12 and secondary windings 14 and 16. The leads of primary winding 12 are connected to 120 Volt AC power via switch 13, causing secondary windings 14 and 16 to develop substantially higher voltages for driving the load. Secondary windings 14 and 16 are drawn as two windings connected in series at a common node 17, but could also be a single secondary winding with a center tap. The center tap or common node 17 of the secondary windings is connected through a ground fault current detection circuit 18 to a path to ground. If detection circuit 18 senses any substantial current flow between node 17 and ground, circuit 18 generates a SHUT DOWN signal on line 20, causing switch 13 to open and remove power from the transformer circuit.
As noted above, a secondary ground fault detector of this kind can be defeated by connecting a mid-point of the load to ground. As seen in FIG. 1, the load connected to secondary windings 14 and 16 includes two series-connected loads 22 and 24, as well as a ground fault path 26 from secondary 14 to ground.
Typically, the neon transformer is a leakage reactance type transformer which exhibits a relatively constant current over a wide range of load impedances. Furthermore secondary windings 14 and 16 are typically virtually identical and therefore produce similar load currents that are opposing (180 degrees out of phase).
Under these conditions, note that the current flowing out of secondary 14 divides and flows into two separate paths. Some current flows through load 22 into ground while the remainder flows through the ground fault path 26 into ground. The current then recombines and flows through ground, through the ground fault current detector 18 and back to secondary 14. The current from secondary 16 flows through the ground fault current detector 18 into ground, through load 24, and back to secondary 16. The current from secondary 16 that flows through the ground fault current detector 18 is similar in magnitude but opposite in direction to the current flowing from secondary 14. This results in little or no net current flow through the ground fault current detector 18, which may not activate despite the presence of a fault current through the fault path 26 which is sufficient to start a fire.
Accordingly, there is a need for a transformer circuit with secondary ground fault detection which cannot be defeated by shorting a mid-point of the load to ground.
An additional common difficulty with transformer circuits is momentary inrush current experienced when power is initially applied. This inrush current can be twenty times greater than the normal steady state operating current, and last ten to twenty AC cycles before normal steady state operating conditions are achieved. This large current can cause nuisance tripping of circuit breakers, and lead to premature fuse or circuit breaker failures. Line voltage sags resulting from high inrush currents can also interfere with other electronic equipment connected to the AC line.
Properly designed transformers, when connected to standard 120 Volt AC power, will achieve a steady state in which the magnetic field intensity and flux density vary with the line voltage within the linear region of the B-H curve of the core, with relatively low loss and low current. When power is removed, this variation will cease. However, if power is suddenly disconnected when the magnetic flux density in the core is near its peak value, a residual amount of magnetic flux density will remain in the core. The core will retain this residual magnetic flux until power is reapplied. The residual magnetic flux is not in itself harmful; however, when power is reapplied, if the initial half-cycle of the applied line voltage generates magnetic flux in the same direction as the residual flux in the core, at the first peak of the applied line voltage, the magnetic flux density in the core will substantially exceed the steady state peak flux density. This can drive the magnetic core of the transformer into the nonlinear (saturated) region of its B-H curve, where the primary inductance of the transformer decreases radically. At this point a very large inrush current will be drawn by the transformer. With successive cycles of the applied line voltage, for as long as ten to twenty cycles, the magnetic core will continue to be driven in one direction into the nonlinear region of the B-H curve of the core, with the extent of the excursion slowly decreasing until steady state operation is achieved. During these ten to twenty cycles, decreasing levels of inrush current will be drawn corresponding to each peak of the applied line voltage, leading to the problems identified above.
Thus, there is a need for a transformer circuit which exhibits reduced inrush currents when power is initially applied.
In accordance with principles of the present invention, the needs described above are met by a power supply circuit incorporating unique fault detection and inrush current prevention features, as well as a diagnostic analyzer for use with a power supply for diagnosing faults.
In accordance with a first aspect of the present invention, the power supply circuit detects ground fault paths, and miswiring of the load, using a test mode in which electrical power is applied to only one power output terminal of the power supply relative to the ground terminal, while substantially no electrical power is applied to the second power output terminal relative to the ground terminal. The power supply includes a ground fault detection circuit connected to a path to ground. If application of power during the test mode causes substantial current flow through the ground path, then there is a ground fault path, or an incorrect connection between the midpoint of the load and ground. An electrical signal of this condition is produced which can be used to notify the operator and/or disable the power supply.
In the specific embodiment disclosed below, the power supply includes a transformer having a primary and a secondary winding, the primary winding being connectable to a source of alternating current electrical power, the first power output terminal of the power supply being connected to a first end of the secondary winding, and the second power output terminal being connected via an electrical switch to a second end of the secondary winding, and a center tap of the secondary winding being connected via the ground fault detection circuit to the path to ground. In this embodiment, in the test mode the switch is opened to disconnect the second end of the secondary winding from the second output terminal.
In an alternative embodiment, the power supply includes first and second transformers respectively having first and second primary windings and first and second secondary windings, the primary windings being connectable to a source of alternating current electrical power, the first power output terminal being connected to a first end of the first secondary winding, the second end of the first secondary winding being connected to a first end of the second secondary winding and the ground fault detection circuit, a second end of the second secondary winding being connected to the second power output terminal.
In this embodiment, in the test mode only the first primary winding is connected to alternating current electrical power, so as to energize only the first secondary winding and first power output terminal. To perform additional testing, only the second primary winding may be connected to alternating current electrical power, so as to energize only the second secondary winding and second power output terminal.
In another alternative embodiment, the power supply may include a clamp winding magnetically coupled to the second secondary winding. In this embodiment, in the test mode the ends of the clamp winding can be shorted together, preventing any substantial energization of the second secondary winding or second power output terminal.
In any of these embodiments, when the ground fault detection circuit detects a fault, the power supply may generate a visual indication of the fault. For example, the power supply may include a light emitting diode which is illuminated when there is a ground fault. Alternatively, the power supply may repeatedly apply and remove electrical power from one or both of the power output terminals to cause the gas discharge load to visually indicate a fault.
In another aspect, the power supply includes unique features for signaling the presence and kind of fault it has detected. The power supply includes a ground fault detection circuit for detecting a ground fault between the load and ground. The power supply is capable of distinguishing between ground faults in at least two different locations, and when a fault is detected, the power supply signals the presence of the fault and its location.
In specific embodiments, the power supply generates a visual indication of the presence and location of the ground fault, for example, by repeatedly applying and removing electrical power from one or both of the power terminals, and/or by illuminating a light emitting diode, and/or by generating an audible signals such as a synthesized human voice. In the specific embodiment disclosed below, a diagnostic analyzer having light emitting diodes and a voice synthesizer, is connected temporarily to a transformer circuit, to form a power supply able to use the light emitting diode and an audible signal to identify the location of a fault. However, in other embodiments, these functions of the diagnostic analyzer may be incorporated into the transformer circuit and thus permanently installed with the transformer circuit.
Another aspect of the invention relates to a diagnostic analyzer for connection to a power supply and diagnosing the presence of a fault. The diagnostic analyzer monitors power flow from the output terminals of the diagnostic analyzer to the input terminals of the power supply, and evaluates the power flow to identify power flow patterns indicative of a fault. When a fault is recognized, the diagnostic analyzer signals the presence of the fault.
In specific embodiments, the diagnostic analyzer circuit generates a visual indication of a fault, e.g., by illuminating a light emitting diode. In addition, the diagnostic analyzer generates a audible signal indicating the fault, e.g., in a synthesized human voice.
The diagnostic analyzer is also used to activate a test mode of the power supply by applying a command signal to the input power terminals of the power supply. For example, the diagnostic circuit rapidly connects and disconnects power to the power supply, to signal the power supply to enter its test mode. Furthermore, the power supply may include circuitry for generating particular power flow patterns to identify a kind of fault, in which case the diagnostic analyzer may recognize the kind of fault from its characteristic power flow pattern.
In another aspect, the invention features a power supply for reducing inrush current incident to application of alternating current electrical power to a primary winding of a transformer. The power supply monitors cycles of alternating current electrical power applied to its power input terminals, and identifies opposite half-cycles of the alternating current electrical power. Using this information, the power supply disconnects and reconnects power to the transformer primary winding in opposite half-cycles of the alternating current electrical power. As a result, residual magnetization in a core of the transformer left upon disconnection of the power input and power output terminals, opposes induced magnetization of the core upon reconnection of the power input and power output terminals, reducing inrush current.
In the specific embodiment described below, the power supply only disconnects power from the primary winding during a predetermined half-cycle, and only connects power to the primary winding during the half cycle opposite to the predetermined half cycle.
In further aspects, the invention features the methods carried out by a power supply and diagnostic analyzer in accordance with the invention, in detecting miswiring and faults, and reducing inrush currents.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.